Over the last several years, significant interest has built up over the use of variable geometry structures to improve performance of vehicles and sub-components. Reconfigurable or morphing structure technology, which permits structural components to undergo large-scale, in-service changes in component geometry, could provide game-changing performance enhancements over broad operating conditions. The benefits of this technology include high performance components, improved and optimized wave interactions (EM, shock, sound, air flow), and deployable systems. Applications that may benefit from such a capability are those needing aerodynamic optimization (such as airplane wings), control surfaces, inlets/outlets, tunable EM surfaces (such as reflectors, antennas, gratings, mirrors) and deployable structures. Existing static shape structures have evolved to be very mass efficient by using exterior panels and surfaces to provide torsional stability and enhanced bending stiffness as in the typical box wing design used for aircraft. In order to preserve this high structural performance in reconfigurable systems, new materials are required which can both accommodate large deformations necessary to achieve performance gains in the component itself, and sustain a large working stiffness which is necessary to retain structural efficiency and low weight.
FIG. 1 is a graph surveying material properties for morphing applications based on intrinsic modulus and reversible elastic strain.
As shown in FIG. 1, various thresholds exist for combinations of strain and stiffness in available materials to build morphing systems. Each of these thresholds is related to the intrinsic mechanism of the material's strength and/or strain reversibility. Traditional structural materials such as metallic alloys and fibrous composites largely obtain their strength from atomic bonding and large deformations only though irreversible dislocation motion. Traditional active materials, based on phase change mechanisms, generally provide large stiffness but are limited in reversible strain magnitude by the relatively small changes in lattice supported by phase changes. Polymers, generally accommodating deformation through reputation (i.e., creep), cannot generally decouple the deformation and stiffness mechanisms and thus result in tradeoff of reversible strain and stiffness that does not provide enough stiffness for structural reconfiguration. Elastomers reside at the far end of the polymer spectrum, providing significant deformation capability through polymer chain unfolding mechanisms, but with very low stiffness that result in significant penalties to structural efficiency.
Given the lack of available constant stiffness materials for morphing applications, variable stiffness materials would be very useful in improving morphing capabilities. Variable stiffness materials potentially can be operated at high stiffness, appropriate for structural efficiency, and operated at low stiffness where large deformations without minimal mechanical energy input can be achieved.
As such, there is a need for variable stiffness materials that can be stiffened and softened with ease, and, more particularly, a polymer-matrix composite or laminate of structural reinforcement elements (constant stiffness component) and a thermosetting polymer (variable stiffness component) to provide exemplary variable stiffness material (VSM) structures. Such structures allow a softened state (or mode) to achieve large reversible deformation with relatively small input energy while maintaining high stiffness in a structural mode. The stiffness and reversible strain of a segmented composite are largely determined by geometrical and spatial variables. For example, the composite stiffness varies with the aspect ratio of the reinforcement segments and with the volume fraction of the stiff component in the overall composite. The maximum reversible strain is limited by the capacity of the matrix material to reversibly accommodate the local shear strain which is a function of the length of the reinforcement segments and the interlaminar spacing (matrix layer thickness). Furthermore, modeling and experiment have shown that the low temperature storage modulus of the VSM composite is sensitive to the gap size, or edge-to-edge distance between reinforcement segments. In order to tailor the properties of the composite to different application requirements, it is desirable to provide a system and method for precisely controlling a three-dimensional distribution of the structural reinforcement elements in the polymer-matrix composite.